Oracle Essentials, 5th Edition

Chapter 8. Oracle Multiuser Concurrency

Sharing data is at the center of all information systems. As systems provide higher and higher levels of functionality, we can sometimes forget that the ability to efficiently share data is the underlying governor of overall system performance. At the same time, database systems must protect the integrity of the data, as the value of that data is directly proportional to the correctness of the data. Database systems must protect data integrity, while still providing high levels of performance for multiuser access. These two forces sometimes conflict and shape some of the core technology in any database system.

Data integrity must always come first. As Ken Jacobs, fondly known as Dr. DBA in the Oracle community, put it in his classic paper entitled “Transaction Control and Oracle7,” a multiuser database must be able to handle concurrently executing transactions in a way that “ensure(s) predictable and reproducible results.” This goal is the core issue of data integrity, which, in turn, is the foundation of any database system.

When multiple users access the same data, there is always the possibility that one user’s changes to a specific piece of data will be unwittingly overwritten by another user’s changes. If this situation occurs, the accuracy of the information in the database is compromised, and that can render the data useless or, even worse, misleading. At the same time, the techniques used to prevent this type of loss can dramatically ...